There are cases where an ink set containing two or three or more black inks each having different carbon black concentration is used for the purpose of obtaining a good black-and-white monochromatic image or a gray scale by an ink jet recording method. For instance, there are cases where a two-color ink set comprising black ink and light black ink is used as an ink set for black-and-white monochromatic image recording. Further, in an ink set for color recording, there are also cases where a four-color ink set comprising black ink, cyan ink, magenta ink and yellow ink (or a six-color ink set additionally comprising light cyan ink and light magenta ink) is made a five-color ink set (or a seven-color ink set) by further adding light black ink thereto in compliance with the requirement of increasing the quality of achromatic part (black, gray and white parts) of a color image. The light black ink is ink having lower concentration of carbon black than that of black ink and the light black ink is used for the purpose of improving the color reproduction of dark colors, e.g., a shadow part, and the gradation of gray.
For example, the stability of gray balance and the metamerism are markedly elevated by using three black inks each having different carbon black concentration. The effect is especially great in gray balance.
That is, when a gray balance is outputted by only one or two black inks, the graininess in a high tone region (a bright region) becomes a problem. For solving this graininess, there is a means of outputting color of a high tone achromatic region by the mixture of cyan ink, magenta ink and yellow ink. However, when a method of outputting color of an achromatic region by a plurality of chromatic colors is adopted, a high technique is required for controlling outputted colors and there is the risk of fluctuation in outputted colors.
The above fluctuation in outputted colors is caused by various factors. One factor of the fluctuation is the unevenness in ink weights among cyan ink (light cyan ink), magenta ink (light magenta ink) and yellow ink. That is, the variations in ink weights results in the fluctuation in outputted colors. For example, when the discharge amount of cyan ink increases by 10% and the discharge amount of yellow ink decreases by 10%, the outputted color shifts to bluish side as a whole. The ink jet printer is of course designed so as to be able to obtain absolutely the same output at any time, however, since ink is a liquid, it is influenced by a change in the environment of the outside. For instance, the viscosity of ink changes by the variation in temperature. In an ink jet method of ejecting ink from an ink jet head by the application of pressure, an ink jet amount is inevitably changed with the viscosity change of an ink liquid. In ink jet printers, a head driver is equipped with temperature compensating function for diminishing such an influence, but it is difficult to completely obviate even a small ink weight change in practice, and there is a possibility that a trace amount of change of ink weight may occur. Further, besides the fluctuation in temperature, humidity is actually a factor to outputted color variation, and recording medium (ink jet medium) also becomes the cause of delicate color change by these environmental influences.
The drawback in the method of outputting the color of a high tone region (a bright region) by a plurality of chromatic colors can be solved by using third black ink having low carbon black concentration. That is, when third black ink having low carbon black concentration is used, a hue of an achromatic color is not influenced even when there arises variation in ink jet amount, as a result, a hue is stabilized.
Black ink is the highest in the visibility of dots. For reducing the dot visibility in a high tone region, it is necessary to reduce the concentration of carbon black contained in the ink. On the other hand, for representing “blackish” surely, it is also necessary to retain carbon black concentration at a certain level or higher. Therefore, ink having the concentration by which dots are invisible to the eye (or difficult to see) and ink having carbon black concentration capable of surely representing a blackish color are necessary. In this point of view, since it is difficult to represent medium gradation with only two kinds of black inks, ink of medium concentration is necessary. As has been described, an ink set comprising three or more kinds of black inks including black ink of low carbon black concentration and black ink of medium concentration is very significant.
However, reports of the results of studies on the effects and defects in cases of performing recording by using ink sets comprising two or three or more kinds of black inks are hardly known up to the present.
For instance, the examples of image-recording apparatus using a plurality of ink compositions each having different concentration and recording heads are disclosed in JP-A-6-226998 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). However, the above publication only discloses the example of a recording apparatus equipped with black, yellow, magenta and cyan inks each having different concentrations of two to four kinds respectively.
Moreover, an image-recording apparatus using a plurality of black ink compositions each having different carbon black concentration in combination with ink sets of color ink compositions and the controlling method thereof are disclosed in JP-A-11-320924. However, the technique disclosed in the above publication aims at forming a monochromatic image of high gradation on the same recording medium separately from a color image, and to use a plurality of black ink compositions for forming the monochromatic high gradation image is merely disclosed in the publication.